1. Field of the Invention
The present invention relates to an improved electromagnetic transducer which includes a rotor and a coil, and which reciprocally transduces kinetic energy generated due to rotation of the rotor to electrical energy generated in the coil. More particularly, the present invention provides an electromagnetic transducer with improved energy conversion efficiency and to electronic devices which employ the improved transducer.
2. Description of the Related Art
Heretofore, an electromagnetic transducer has been utilized which transduces changes in magnetic flux due to rotation of a rotor into electrical energy, or conversely transduces electrical energy into rotary power. Examples of such an electromagnetic transducer employed in a multipolar power generator are described in, e.g., Japanese Unexamined Patent Publication Nos. 8-75874 and 9-211152, and in a paper in Proceedings of The Swiss Society of Chronometry reported by Asulab S. A. on Oct. 2, 1997.
Each of these multipolar power generators comprises a plurality of magnets rotatably supported about a predetermined axis and arranged on a plane perpendicular to the predetermined axis with N and S poles arrayed alternately in the direction of rotation, a rotor having a back yoke made of soft iron supporting the plurality of magnets, and a coil made up of windings and arranged near the rotor. With this type of multipolar power generator, when the rotor is rotated with a mechanical energy source, such as a barrel main spring, the magnetic flux near the coil is changed with the rotation of the rotor, and an induced current flows through the coil windings by electromagnetic induction.
The above-described multipolar power generator is suitable as a power supply source for, e.g., a speed-governing control circuit of an electronically controlled watch or the like because inductive electromotive forces can be generated with stability by such an arrangement where the plurality of magnets are arranged with N and S poles arrayed alternately in the direction of rotation. Also, compared with a conventional power generator which comprises a rotor, a metal-made core arranged near the rotor, and a stator including a coil wound over another portion of the core, a power generator producing no iron loss and having higher efficiency of power generation can be realized by not employing a metal core. Alternatively, by not arranging a metal-made core near the rotor, a power generator having a small cogging torque and very excellent startability can be achieved.
However, the following problems are experienced in the electromagnetic transducer described above.
(1) Because the rotor includes the back yoke made of soft iron and the plurality of magnets arranged on the back yoke as described above, the rotor has increased weight. Accordingly, a rotor shaft supporting those components is required to have a diameter of sufficient size and durability to accommodate the increased weight. An increase in diameter of the rotor shaft increases the contact area between the shaft, a main plate and a bearing support seat which hold the rotor shaft. Correspondingly, the rotation of the rotor is restricted and the power generation efficiency of the multipolar power generator is reduced.
(2) To rotate the more weighty rotor with stability, a bearing portion formed in the main plate, etc. must be an anti-vibration bearing, which has been used in conventional mechanical watches, for the purpose of improving anti-vibration ability and anti-impact ability. This results in increased thickness of the multipolar power generator.
The above problems are also similarly encountered when an electromagnetic transducer is employed as a motor using electrical energy as a power source.